Hybrid electric vehicles (HEVs) combine the internal combustion engine of a conventional vehicle with the battery and electric motor of an electric vehicle. This combination offers the driving range and rapid refueling features to which consumers are accustomed with conventional vehicles, while achieving improved fuel economy and lower emissions.
Typical HEV designs such as those already on the market and those which will be introduced shortly, are so-called “parallel” configurations. In the parallel HEV, the battery powered motor is principally used to boost engine torque for hill climbing and high acceleration demands. When boost torque is not required, the engine drives the electric motor as a generator to recharge the battery. The motor is also driven as a generator during braking events, thus, relieving thermal loading of the conventional friction brakes and enabling recovery of vehicle kinetic and potential energy which is returned to the battery.
The battery “pack” of a typical HEV consists of one or more “modules” of series-connected “cells.” Nickel cadmium (NiCad) and nickel metal hydride (NiMH) cells have been successfully employed in recently introduced HEVs while higher performance lithium ion (Li-ion) cells are envisioned for future generation designs.
Desirable attributes of battery cells for HEV applications are high-peak specific power, high specific energy at pulse power, fast charge acceptance to maximize regenerative braking utilization, and long calendar and cycle life. Achieving a favorable HEV battery pack lifetime requires some means to monitor cell state of charge (SOC) and control of cell charging and discharging to assure that all cells in the pack are well “balanced” or “equalized,” for example, at a nominally uniform state of charge. The development of means to achieve affordable and reliable balanced cell operation, especially for newer Li-ion cells, has presented significant technical challenges Lithium ion batteries are now widely used in laptop computer and cell-phone products because of their high specific energy. They also have high specific power, high energy efficiency, good high-temperature performance, and low self-discharge. Components of lithium ion batteries could also be recycled. These characteristics make lithium ion batteries desirable for HEV applications. However, to make them commercially viable for HEVs, further development is needed to improve calendar and cycle life and cost.